Study On Microstructure And Properties Of DP680 Hot Rolled Dual Phase Steel

In recent years,automotive materials have been developed in the weight reduction,energy saving and high safety,the high-strength steel plate has been applied to the automobile manufacturing industry.The hot rolled dual phase steel has excellent comprehensive properties,it is an ideal steel of the automobile structure.In terms of strength level,the hot rolled dual phase steel produced and applied at home and abroad is mainly DP540-DP600.The market demand for hot rolled dual phase steel of higher strength level is imminent,however,there are a few of researchs and production reports of 680 MPa hot rolled dual phase steel.In terms of thickness grades,the market has a high demand of the thicker(>10 mm)high-strength hot rolled dual phase steel,which is mainly used in the heavy trucks wheel spokes and can effectively improve the safety performance of heavy trucks.In this research,the dual phase steel with chemical composition of 0.075%C-0.14%Si1.38%Mn-0.63%Cr was used as the tested steel to study the microstructure and properties of 680 MPa hot rolled dual phase steel.This research made the theoretical research on the high temperature austenite deformation behavior,the continuous cooling transformation behavior of austenite,the process parameters during the deformation and cooling process of the tested steel,and the exploration of hot rolled process in laboratory and industrial field trial production.The main findings are as follows:(1)The deformation resistance and dynamic recrystallization behavior of tested steel were studied by thermal simulation experiment.The results showed that when the deformation rate was 5?20 s-1 and the deformation temperature was 820?900?,the true stress-true strain curves were dynamic recovery type,and the deformation resistance model of tested steel was established,the model accuracy was very high.When the deformation rate was 0.05?0.1 s-1 and the deformation temperature was 1000?1150?,the true stress-true strain curves were dynamic recrystallization type,and the dynamic recrystallization model of tested steel was established.(2)The continuous cooling transformation behavior and the variation of microstructure of tested steel were studied through the dynamic transformation instrument.The results showed that there were four transformation zones in the CCT curves when the cooling rate was in the range of 0.5?40?/s.When the cooling rate was small,the transformation of ferrite and pearlite will occur.When the cooling rate was large,the transformation zone of ferrite will gradually decrease until disappeared,and the transformation zone of martensite will appear and bainite exist in the whole cooling rate range.With the increase of cooling rate,the initial transformation temperature of ferrite decreased gradually,the fraction of ferrite was decreased,and the grain was refined.(3)The effects of process parameters on the microstructure evolution of tested steel was studied through thermal simulation experiment.The results showed that with the increase of austenitizing temperature and time,austenite grains growed gradually,and the effect of heating temperature on grain growth was obviously.Low deformation temperature and large deformation are beneficial to ferrite transformation.When the total cooling time was the same,the faster the cooling rate,the more the ferrite fraction.The "nose tip" temperature of ferrite transformation of tested steel is about 660?,the ferrite fraction increased with the increase of the holding time.When second stage cooling rate was?20?/s and the coiling temperature was ?400?,the lath martensite appeared in the microstructure.(4)The effects of different parameters on the microstructure and properties of tested steel during hot rolled was studied through hot rolled experiment in laboratory.The results showed that with the decrease of the intermediate temperature,the ferrite fraction increased and the yield strength and tensile strength decreased gradually.Both the thin and thick hot rolled plates met the requirements of 680 MPa hot rolled dual phase steel.When the coiling temperature was 545?,there was the yield plateau in the tensile curve.When the coiling temperature was ?410?,there was not yield plateau,and the microstructure was a typical ferrite+martensite dual phase structure.Increasing the second stage cooling rate is conducive to the improvement of tensile strength.Under the three-stage cooling mode,the ferrite fraction was more and the size was smaller,the martensite distribution was more uniform,and the yield ratio was significantly reduced.Through field trial production,the microstructure and mechanical properties of the hot rolled plate were tested,which met the requirements of DP680 standard.